Stitching refers to the alignment of the printed image data from multiple jetting modules for the purpose of creating the appearance of a single page-width linehead for printing on a print medium 16. For example, as shown in FIG. 1, seven jetting modules 2, each being three inches in length, can be aligned together to form a page-width linehead 4 spanning twenty-one-inches in the cross-track direction 23. The jetting modules 2 can also be interchangeably referred to as “printheads” within the context of the present disclosure. Dashed lines are used to show the print boundaries 3 of the first-row jetting modules 2 as the print medium 16 moves past the linehead 4 in the in-track direction 22, passing from the first row 7a of jetting modules 2 to the second row 7b of jetting modules 2. The page-width image data is processed and segmented into separate segments to be printed with each jetting module 2, and then a segment is sent (with an appropriate module-to-module time delay to account for the staggered separation of the jetting modules 2) to the print nozzles 6 of each jetting module 2 for printing. The result of proper stitching is a continuous print band 60 that spans the width of the linehead 4 with no artifacts at the seams 64 between the swaths 62 of pixels 66 printed by the separate jetting modules 2 as shown in the lower portion of FIG. 1.
However, though it may be anticipated that the module-to-module alignment may be very good, mechanical tolerances may be difficult to consistently maintain, and therefore alignment will often not be perfect. Moreover, even if the jetting modules 2 are perfectly aligned, differences in the nozzle aim between jetting modules 2 may make them appear to be misaligned in the printed output. Consequently, this type of conventional, multi-segment linehead configuration suffers from the drawback that the pitch of the output lines along the print boundaries 3 between adjacent jetting modules 2 is irregular and thereby causes lines of lower (if the jetting modules 2 are too far apart) or higher (if the jetting modules 2 are too close together) density to appear at the print boundaries 3 between each jetting module 2, and thus impairs the quality of the printed pattern of the output. On the print medium, such misalignment in the cross-track direction 23 typically produces a gap or “white-line” artifact 8a (as shown in FIG. 2A) or an overlap or “dark-line” artifact 8b (as shown in FIG. 2B) at the seam 64 between two swaths 62a and 62b. 
With a view to overcoming the presence of visible gaps or bands in the printed image, U.S. Pat. No. 7,118,188 (Vilanova et al.) teaches deliberately positioning the jetting modules (i.e., the printhead dies) of an inkjet printer with a small overlap, specifically no more than a few times the nozzle spacing. As a result of the redundancy of nozzles in the region where adjacent jetting modules overlap, this gives flexibility for compensating for gaps or bands produced by inaccuracies in locating the jetting modules and thus in setting the overlap dimension. Although, in an ideal case, 100% of the required amount of ink (maximum) would be printed by only 50% of the nozzles of each jetting module in the overlap region, in practice more or fewer of the nozzles may be fired to compensate for imperfections. For example, if the overlap is less than intended, the production of a gap is avoided by firing some of the nozzles which would not be fired in the ideal case.
A printing mask is a means for selectively masking off certain nozzles (i.e., preventing the nozzles from firing even if printing instructions for those nozzles should include an instruction to fire). The aforementioned U.S. Pat. No. 7,118,188 further discloses a method of adding stitching masks to the printed image content, where artifacts in the printed image caused by the printing nozzles in the overlapping region are removed, either by (a) measuring the width of the band produced in the overlapping region and selecting an appropriate stitching mask for subsequent printing operations, or by (b) printing out a test pattern in which areas corresponding to a range of stitching masks are printed out and the optimal mask is selected for subsequent printing operations. The stitching mask is then added to, or superimposed on, the printing masks to ensure that the required correction is made independently of the content to be printed.
The aforementioned U.S. Pat. No. 7,118,188 further discloses that the target may comprise an array of target patches overlapping the boundaries between the jetting modules and including a range of stitching masks. The magnitudes of the boundary artifacts are then assessed, either by a user of the machine or automatically by an optical sensor/scanner system. In the first option, a user visually examines the patches in each row and selects the one with the better area fill uniformity at the printed region corresponding to the jetting module boundary. The corresponding stitching mask is then applied to that jetting module boundary in subsequent normal printing operations. In the second option, an optical sensor moves over all the patches detecting the boundary artifact level. The most appropriate stitching mask is then selected for each jetting module pair and supplied to a printer control system, where the masks will then be used in subsequent normal printing operations.
In relation to page-width thermal printers, U.S. Pat. Nos. 4,977,410 and 5,450,099 each disclose a thermal line printer including a plurality of staggered linear head segments arranged in a pair of parallel rows such that the head segments partly overlap with each other in overlap regions near the ends of each head segment. In commonly-assigned U.S. Pat. No. 5,450,099 (Stephenson et al.), the print data in the overlap region is interleaved to eliminate boundary artifacts at the juncture between segments. In U.S. Pat. No. 4,977,410 (Onuki et al.), the initial assignment of image bit data to a head segment in the overlap region is shifted lengthwise to accommodate for boundary artifacts at the juncture between head segments.
In relation to a carriage-type printer wherein a printhead is attached to a carriage that is reciprocated to print one swath of information at a time on a stationary print medium, U.S. Pat. No. 6,663,206 (Taylor) discloses methods for masking stitch errors between adjacent swaths laid down by operation of such a printer. In contrast with the aforementioned examples of page-width printers that utilizes a linehead including an array of stationary printheads, after each swath is printed by the carriage-type printer the print medium is stepped a distance equal to the height of the swath so that the next printed swath overlaps the pixels from the last line of the previously printed swath. When a controller determines that a stitch joint error will occur based on the current relative location between the printhead and the location of the previous swath on the print medium, the location of the next swath is adjusted relative to the position of the previous swath to eliminate the stitch joint error.
According to U.S. Pat. No. 6,663,206, the data is shifted in the printhead so that the data for the next swath is aligned within a predetermined pixel accuracy to the measured paper position (e.g., by having a later nozzle fire the pixel data originally set to be fired by the first nozzle of the printhead). In addition, the remaining stitch joint error is covered up by modifying the pixels at the stitch interface. In one example, the pixels created in the region between the last line of the previous swath and the first line of the next swath can be a duplicate line of either the last line of the previous swath or the first line of the next swath, where the size and/or density of the pixels can be changed. In another example, for situations where the stitch error is less than a pixel, in addition to shifting the data and firing the information set to be printed, the controller will also fire a line of fill pixels from the nozzle prior to and immediately adjacent to the first-fired nozzle. The purpose of a fill pixel is to bridge the gap between a printed pixel from the last fired nozzle of the previous swath and a corresponding adjacent printer pixel that will be formed when the first line of pixels is formed by the nozzle that will be used for the first line of pixels for the next swath. According to U.S. Pat. No. 6,663,206, the fill pixels create a printed image having more uniform continuity and density. The fill pixels are not produced for all of the pixels located in the last line of the previous swath. Instead, the fill pixels are produced when a printed pixel is located in the same position in both the previous swath and the next swath. The fill pixels can also be at a reduced size and/or density.
Stitch joint errors in a drop-on-demand carriage-type system can be the result of a gap between the drop of one swath adjacent the stitch joint and the drop of an adjoining swath adjacent the same stitch joint. As explained in U.S. Pat. No. 6,663,206, the gap is usually caused by difficulties in producing adjacent swaths close enough together to mask this apparent error, and the correction must be produced on-the-fly during a production run. In contrast, as also explained in U.S. Pat. No. 6,663,206, a page-width printer includes a stationary printhead having a length sufficient to print across the width or length of the sheet of print medium. The print medium is continually moved past the page-width printhead in a direction substantially normal to the printhead length and at a constant or varying speed during the printing process. Thus, it would be understood that a page-width printer would avoid the need for on-the-fly corrections between swaths during a production run.
Even with such algorithms, under some conditions an artifact can still form at the seams. In particular, visible artifacts can be seen under print conditions in which the print medium 16 drifts laterally back and forth during printing. This is illustrated in FIG. 3, which shows a portion of a linehead 4 and a portion of a print band 60 under the condition of a lateral drift in print medium 16 producing a slight skewing of the print medium 16. This skewing of the print medium 16 causes the print swath printed by the upper jetting module 2 to be shifted to the left as the print medium 16 advances to the lower row of jetting modules 2. This shift of the center print swath produces a larger than normal pixel spacing at seam 64a and a smaller than normal printed pixel spacing at the seam 64b. As the print medium 16 drifts back and forth, it can cause the apparent pixel spacing at the seams to be modulated. While the different stitching algorithms described above can conceal the seams when the pixel spacing at the seams is static, they cannot conceal the seams when the pixel spacing at the seams modulates.
Even if the artifacts at the seams are quite small, they might be detected by an observer because the human vision system is particularly effective in detecting a collection of image artifacts when the artifacts are aligned along a line. In recognition of this, European Patent Application EP0034060 (Kockler) discloses a stitching algorithm in which from one row of pixels to the next, the stitching boundary is randomly or cyclically shifted around within a 16-pixel overlap region. By dispersing the position of the stitch boundary in this manner, the stitch artifacts produced by variations in the printed pixel spacing across a seam between swaths are less noticeable to the human observer.
However, if there are in-track shifts between adjacent swaths the randomly dithered stitch boundary can produce a different type of print artifact. This is illustrated in FIG. 4, which shows a portion of two adjacent print swaths 62a, 62b, with a dithered stitch boundary 68. The pixels 66 of the two different swaths are denoted by differing hatch patterns. The dithering of the stitch boundary 68 creates a seam comprising interdigitated fingers 74 printed by the two swaths 62a, 62b. Swath 62a is displaced both in the cross-track direction 23 and the in-track direction 22 relative to a proper alignment with swath 62b. The in-track placement error causes horizontal gaps 70 to appear between some of the interdigitated fingers 74, and excessive coverage regions 72 to appear between others of the interdigitated fingers.
To reduce the visibility of gap and excessive overlap artifacts produced by the dithered stitch boundary, U.S. Pat. No. 6,357,847 (Ellson et al.) teaches randomly shifting the position of the stitch boundary, not on a pixel row-by-pixel row basis, but rather shifting the position of the stitch boundary every Nth pixel row, where N>1. Such a change reduces the number of gaps and excessive coverage regions produced by the dithered stitch boundary by a factor N, to reduce the visibility of these artifacts.
U.S. Patent Application Publication 2004/0218200 (Ebihara) discloses an alternate approach to reducing the visibility of the artifacts produced by the random dithering of the stitch boundary by altering the weighting profile for the placement of the stitch boundary on any given step. By decreasing the probability for placement of the stitch boundary at the extremes of the overlap region, it reduced the probability of large steps in the placement of the stitch boundary. By doing this, the average length of the horizontal gaps and the excessive coverage regions is reduced, thereby reducing the visibility of these artifacts.
Even with such stitching algorithms, there remains a need for improved stitching algorithms to reduce or eliminate visible artifacts at the swath seams.